It may be desirable in some technical fields to measure the composition of a fluid flowing through a fluid conduit. For example, in the oil and gas industry, it may be desirable to measure the composition of a two- or three-phase fluid produced from an oil or gas well. In particular, it may be desirable to accurately measure the proportion of water by volume in the liquid phase, also known as the water-cut, of a two- or three-phase fluid produced from an oil or gas well.
It is known to use a meter which comprises a cavity resonant at radio frequencies to determine a composition of a fluid and, in particular, to determine the water-cut of a two- or three-phase fluid from measurements of one or more properties of one or more resonant electromagnetic modes of the cavity. However, such resonant electromagnetic measurements can become more difficult or less accurate at higher water-cut values in the range of 50%-100% due to the absorption of electromagnetic energy at radio frequencies in water and, in particular, in saline water. This is particularly the case for water-continuous fluid flows where the water cut is sufficiently high that the liquid part of the fluid flow can be regarded as water containing enclosed areas of a liquid other than water such as enclosed areas of oil.